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Methods

Seventy eyes of 35 consecutive refractive surgery candidates were included in this
study. The mean age of the subjects was 26.42 ± 6.95 years, the average CA was −1.17
diopters (D; SD 0.64; range −0.2 to-3.3D), All subjects in this study were WTR CA.
34 eyes were in the normal CA group with a mean CA was −0.67 ± 0.28D, 36 eyes were
in the high CA group with an average CA of −1.65 ± 0.49D. All subjects underwent ophthalmic
examination and imaging with the Cirrus HD OCT.

Results

No significant difference was noted in the average cup-to-disk ratio, vertical cup-to-disk
ratio and cup volume (all P values > 0.05). Compared with the normal CA group, the high CA group had a larger
disc area and rim area, thinner RNFL thickness in the temporal quadrant, and the superotemporal
and inferotemporal peaks were farther to the temporal horizon (All P values < 0.05). There were no significant differences between the two groups in global
average RNFL thickness, as well as superior, nasal and inferior quadrant RNFL thickness
(all P values > 0.05).

Conclusions

The degree of with-the-rule CA should be considered when interpreting ONH parameters
and peripapillary RNFL thickness measured by the Cirrus HD OCT.

Virtual slides

Keywords:

High myopia; Corneal ASTIGMATISM; Optical coherence tomography

Introduction

Astigmatism is a worldwide common ocular disorder. Total astigmatism is mainly driven
by corneal astigmatism(CA), which occurs due to an irregular shape of the cornea.
In eyes with astigmatism, retinal images can be distorted. Langenbucher et al.
[1] reported that the retinal image was distorted to an ellipse, and the image size could
vary according to the axis of astigmatism assessed with computer-based methodology
in astigmatic eyes.

Optical coherence tomography (OCT) can provide imaging of ocular structures by a noninvasive
method, It is widely used in clinical and scientific ophthalmology to obtain high-resolution
cross-sections of the retina images. The thickness of the retinal nerve fiber layer
(RNFL) and optic nerve head (ONH) parameters can be measured by OCT. Evaluation of
these parameters is essential, since the thickness of the RNFL may be effected in
various diseases. For example, the RNFL becomes thinner in glaucoma and optic atrophy,
whereas it is thicker in papilledema.

Many studies have reported the effect of refractive error changes induced by refractive
surgery or contact lenses on RNFL thickness measured by OCT
[2-4], while little is known about the effect of cylindrical refractive error (astigmatism)
on RNFL and ONH parameters measured by OCT. The purpose of this study was to evaluate
the influence of corneal astigmatism on the peripapillary RNFL thickness and ONH parameters
obtained by Cirrus HD spectral-domain OCT (Cirrus HD OCT; Carl Zeiss Meditec, Dublin,
CA, USA) in Chinese subjects with high myopia.

Material and methods

Subjects

70 eyes of 35 consecutive refractive surgery candidates with spherical equivalent ≥ −6
diopters (D) were recruited for the study. Ethical approval for the study was obtained
from the local medical ethics committee. All subjects were volunteers and informed
consents were obtained.

Each subject underwent a full ophthalmic examination, which included measures of visual
acuity, refraction, intraocular pressure (IOP) by a noncontact tonometer. Axial length
measurements were obtained in each eye with the IOL Master (Carl Zeiss Meditec, Inc,
Dublin, CA), CA measurements were obtained by a Topolyzer (Allegretto Wave Topolyzer,
Germany), optic nerve head evaluation was performed with a 90-D lens, and peripapillary
RNFL thickness and ONH parameters were measured with the Cirrus HD OCT (Cirrus HD
OCT; Carl Zeiss Meditec, Dublin, CA). The peak locations of the superotemporal and
inferotemporal areas were evaluated by the RNFL TSNIT curve of the Cirrus HD OCT.
The peak locations, which were measured by the RNFL TSNIT curve, were translated to
units of degrees by multiplying 360/256. For example, the superior peak location of
40 in the TSNIT curve was translated to 56.25 degrees (40 × 360/256 degrees). This
means that the thickest superior RNFL was located at the point 56.25 degrees away
from the temporal horizontal meridian. We defined α angle as the angle between the
horizontal meridian and superotemporal peak location by clockwise rotation, and the
angle between the horizontal meridian and inferotemporal peak locations by counterclockwise
rotation were defined as β angle (Figure
1).

Figure 1.An example of a measurement of the retinal nerve fiber layer characteristics in an
right eye with a-6.625D of spherical equivalent, a -1.6 of CA and a 26.42mm of axial
length: (a) Fundus photograph of the optic disc. Dotted line represents imaginary horizontal meridian; (b) the peak locations at the superior and inferior area were 40 and 212, respectively;
(c) the peak locations were translated to units of degrees by multiplying 360/256. Angles
between the horizontal meridian and the superotemporal / inferotemporal peak locations
were defined as the α (superotemporal) and β (inferotemporal) angles, so RNFL peak
locations of this eye were α = 40 × 360/256 = 56.25(degree), β = 360-212 × 360/256 = 61.88(degree).

The individuals were included if they had the following: best corrected visual acuity
of 20/20 or better, an intraocular pressure (IOP) lower than 21 mmHg in either eye,
CA as a with-the-rule (WTR) astigmatism, a healthy ONH without glaucomatous damage
(i.e., no disc haemorrhage, notching or thinning of the neural rim).

Those with a history of severe ocular trauma, intraocular or refractive surgery or
any ocular or neurological disease that could have affected the ONH or RNFL were excluded
from the study. Subjects with evidence of macular disease or peripapillary atrophy
extending more than 1.73 mm from the center of the optic disc or with glaucoma or
an IOP higher than 21 mmHg in either eye were also excluded. In addition, participants
with a history of systemic diseases including hypertension and diabetes were excluded.

We assigned astigmatic types as defined in Katz and Kruger
[5]: with-the-rule (WTR) astigmatism was assigned if the plus cylinder axis was within
30° of 90°, against-the-rule (ATR) astigmatism was assigned if the plus cylinder axis
was within 30° of 180°, and the others were assigned as oblique. Astigmatism was defined
as equal to 1.0 D, as in multiple previous studies
[6,7].

Astigmatism

CA was measured with Topolyzer (Allegretto Wave Topolyzer, Germany) and the total
astigmatism was measured by a refractor keratometer (Topcon KR-8800 Auto Refractor).

OCT imaging

After pupillary dilation to a minimum diameter of 5 mm, the eyes of the subjects that
satisfied the study criteria were scanned using the Cirrus HD-OCT system with software
version 5.0. All the scans had signal strength of at least 6 and all measurements
were taken by a single, well-trained examiner. The superior clock hour was 12 o’clock
and the others were assigned accordingly in a clockwise manner in the right eye and
counterclockwise in the left eye.

Statistical analysis

Statistical analyses were performed with commercially available software (SPSS ver.
17.0; SPSS Inc, Chicago, IL). The total average and mean clock hour RNFL measurements
were compared between the two groups with an independent t-test. Correlations between RNFL parameters and astigmatism were examined by linear
regression analysis and expressed as the Pearson coefficient of correlation (r). A p value <0.05 was considered statistically significant.

Of the 70 subjects, 36 eyes were classified as high astigmatism (≤ − 1 D of CA; mean-1.65 ± 0.49D),
and 34 eyes were classified as normal astigmatism (> − 1D of CA; mean-0.67 ± 0.28D).
Characteristics of the two groups are listed in Table
1, no significant differences were found for age, sex, axial length and spherical equivalent
between two groups. The distribution of ONH parameters and RNFL thicknesses were listed
in Table
2.

Table 2.Comparisons of ONH parameters in different astigmatism groups ()

Table
3 and Figure
2 showed the high CA group had significantly thinner RNFLs than the normal astigmatism
group in the temporal, 2 o’clock, 9 o’clock and 10 o’clock sectors. The superotemporal
and inferotemporal peak locations were farther temporally located in eyes with higher
CA.

Table 3.Comparisons of RNFL thickness and peak locations in different astigmatism groups ()

Discussion

The optical coherence tomographer is a modern imaging device designed to measure the
RNFL and ONH parameters in a noncontact and noninvasive manner. RNFL measurements
have been reliable and reproducible, and newer versions of optical coherence tomographers
based on spectral domain technology that provide higher resolution and faster scanning
speeds have been developed
[8,9].

It has been reported that many factors, including refractive error, axial length,
myopic optic disc tilt, eccentric scan location, and head tilt during the examination
can affect the OCT measurements
[10-13]. Lee et al.[14] reported that refractive error changes induced by wearing soft contact lenses of
eight diopters without astigmatic power could affect RNFL thickness measured by a
Cirrus HD OCT. They considered the RNFL thickness was underestimated in eyes with
increasing negative refractive error, while it was overestimated with increasing positive
refractive error. Therefore we hypothesize that, not only spherical refractive error,
but also cylindrical refractive error can affect OCT measurements.

Our study showed that CA influenced spectral-domain OCT measurements of both RNFL
thickness and ONH parameters. Eyes with higher CA had a larger disc area and rim area,
thinner temporal RNFL thickness and farther temporally positioned superotemporal and
inferotemporal peak locations of RNFL thickness. The high CA group had significantly
thinner RNFL thickness than the normal astigmatism group in the 2 o’clock, 9 o’clock
and 10 o’clock sectors (Figure
2).

Our results showed an intriguing finding that had not been reported previously. To
date, the mechanism for changes in RNFL thickness and ONH parameters induced by astigmatism
is not clear, however, possible explanations are as the followings: In high myopes,
the optic disc is usually inserts obliquely. Once the optic disc tilts temporally,
the nasal half of the optic disc elevates anteriorly, and the temporal half of the
optic disc depresses posteriorly
[15-17]. The CA may enhance the magnification effect among high myopes, which may be result
in the disc area and distance from the disc rim border to the disk front surface were
exaggerated. Such changes can lead to differences in reflectivity or backscatter detected
by the OCT and subsequent differences in the RNFL thickness measurements.

These findings are ascribable to CA induced ocular magnification. The relationship
between the measurement of the OCT image and the size of the actual fundus dimension
can be expressed as t = p ·q ·s according to the Littmann formula
[18], Where t is the actual fundus dimension, p is the camera magnification factor in the OCT imaging system, q is a magnification factor related to the eye, and s is the measurement in OCT. Various methods have been introduced to estimate factor
q based on the ametropia, keratometry, and or axial length
[19]. Although one can input the patient’s axial length and spherical equivalent in OCT,
the effect of astigmatism has not been considered. Hwang et al[3] suggested that when the degree or axis of astigmatism changes, RNFL thickness measurement
can be affected by changing the scan distance from the optic disc. All subjects in
this study were WTR CA and the plus cylinder axis was within 30° of deviation from
the 90° meridian. The maximum power was in the vertical meridian, the result for in
the optic disc was vertically oval, and the scan circle was farther from the optic
disc in the horizontal meridian. Thus, the measurement of RNFL thickness between two
groups, using the same-sized scan circle, might be misleading because the RNFL thickness
decreases at increasing distances from the optic disc
[20]. There was a tendency for the RNFL thickness in the temporal and nasal regions to
become thinner, even though the RNFL thickness of the nasal region was not statistically
different between two groups (P = 0.067).

In this study, the sample size may be inadequate to reveal a statistically significant
correlation between total astigmatism and the temporal / nasal quadrant average RNFL
thickness. Further studies are needed to clarify this point.

In conclusion, we found that high corneal astigmatism with the rule influences the
measurements of both RNFL thickness and ONH parameters by the Cirrus HD OCT. Eyes
with higher corneal astigmatism had a larger disc area and rim area, thinner temporal
RNFL thickness and farther temporally positioned superotemporal and inferotemporal
peak locations of the RNFL in high myopes. Therefore, the degree of corneal astigmatism
with the rule influences should be considered when interpreting the ONH parameters
and peripapillary RNFL thickness measured by the Cirrus HD OCT in high myopes.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

LL participated in the study design, reviewed the literature, collected the clinical
data, and drafted the manuscript. JZ provided the conception and design of the study
and reviewing the manuscript. HH collected the clinical data and selected the material.
J-gY took part in the study design and performed the statistical analysis. S-rC participated
in collected the clinical data. All authors have read and approved the manuscript.